1. Field of the Invention
This invention relates generally to capacitive elements in which the capacitance can be varied by an applied bias voltage.
2. Description of the Related Art
Circuit elements which have a variable capacitance are a staple component in the design of electronic circuits. In one approach, the capacitance is adjusted by a bias voltage applied to the capacitive element. These voltage-variable capacitors (varactors) can be created using a number of different technologies, including technologies based on thin-film ferroelectric materials.
In the thin-film ferroelectric approach, a thin film of ferroelectric material is sandwiched between conducting electrodes. Examples of suitable ferroelectric materials include barium titanate, strontium titanate, and composites of the two, for example barium strontium titanate (BST). The capacitance value of this structure varies with the applied electric field due to the nonlinear electrical polarization characteristics of the ferroelectric film. The applied electric field is approximately given by E=V/d, where E is the electric field, V is the voltage applied across the varactor, and d is the thickness of the ferroelectric film. In practice, electric field strengths of up to 1 MV/cm are required to achieve useful capacitance variations, depending on the specific material composition. However, increasing the field strength further leads to device failure or reliability concerns.
In many applications, the varactor is primarily used to process AC signals and a DC bias voltage is applied across the varactor to set the capacitance of the varactor. The capacitance is tuned by varying the DC bias voltage. However, if the application is limited to low DC voltages (e.g., in battery powered applications) and it is also desirable to tune the varactor over a large range of capacitances (i.e., high xe2x80x9ctunabilityxe2x80x9d), then the dielectric film typically must be quite thin in order to achieve the required electric fields. But thin dielectric films result in low breakdown voltages and poor AC power handling. In many circuits such as power amplifiers for wireless applications, the peak AC voltage applied across the varactor can significantly exceed the DC bias voltage. For example, in current cell phones, the battery voltage is typically around 3.5V and the battery produces the DC bias voltage. Therefore, the DC bias voltage typically is limited to 3.5V or less. However, the total voltage (AC+DC) can reach over 7V.
One common approach to increasing the breakdown field in ferroelectric films has been to lightly dope the films with one or more materials. For example, Ti, Mg, Mn, and Zr have been used in BST films to increase the breakdown field. The disadvantage of this approach is that the composite material often has a greatly reduced capacitive tuning for a given applied voltage. This forces the designer to use even thinner films, thus exacerbating the breakdown issue and counteracting gains resulting from the dopants.
Another problem with varactors is that varactors are used as one component in a larger circuit. However, the DC bias voltage typically is applied at the same two terminals which are connected to the external circuit. As a result, the DC bias voltage and corresponding biasing circuitry may not be isolated from the external circuit and interference between the two may result.
Thus, there is a need for capacitive elements which are tunable using low DC bias voltages but which are also capable of handling high AC voltages. It would also be beneficial for the DC bias voltage and circuitry to be isolated from any external circuit in which the capacitive element was used. Capacitive elements based on ferroelectric thin-films typically would have the added advantages of small size, low cost and suitability for mass production.
The present invention overcomes the limitations of the prior art by providing a capacitive element based on two or more varactors. The varactors are configured so that they are coupled in series with respect to an applied AC signal, thus increasing the AC power handling capability since the total AC voltage swing is divided among all of the varactors. The varactors are coupled in parallel with respect to an applied DC bias voltage, thus maintaining high capacitive tunability with low DC voltages since each varactor experiences the full DC bias voltage.
In one embodiment, the voltage-variable capacitive element includes N (where N greater than 1) varactors that are coupled in series to form a chain. The N+1 nodes in the chain shall be referred to as junction nodes. The capacitive element also includes a first AC node and a second AC node for receiving an AC signal. The first AC node is coupled to the first junction node and the N+1th junction node is coupled to the second AC node. The capacitive element further includes a first DC bias node and a second DC bias node for receiving the DC bias voltage. The first DC bias node is DC coupled to the odd numbered junction nodes and the second DC bias node is DC coupled to the even numbered junction nodes. In some implementations, the DC bias node(s) are coupled to the junction node(s) by AC blocking circuit elements, such as high impedance (i.e., AC blocking) resistors or inductors. In this way, the DC biasing circuitry is isolated from the AC signal. In another aspect of the invention, the AC node(s) are coupled to their respective junction node(s) by DC blocking capacitors, thus isolating the DC bias voltage from any external circuit.
The capacitive elements may be implemented using a wide variety of technologies. For example, discrete components may be used to implement some or all of the capacitive elements. However, the varactors preferably are thin-film ferroelectric varactors and the capacitive element preferably is integrated on a single substrate with the varactor.